The criteria that come to mind are not exclusive to horror novels, so I'm a little confused and annoyed (annoyed because I find horror novels scary, and yet just cannot define it logically when I try to distill the reasons in words). Some of the criteria that come to mind are:

This is really a discussion question; there is no one right answer. (You ended with "Any ideas?" which is pretty bluntly an invitation to discussion.) People have varying sensitivities and triggers. Not every Stephen King novel is "horror," and you could argue that not every Tom Clancy novel is a "thriller." Stack Exchange is about concrete, answerable questions with essentially objective answers, and this is too subjective.
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Lauren IpsumMar 20 '14 at 11:24

You're right. I'm starting to doubt whether there is an answer to this at all. I don't know; this issue has just been nagging at me, and I thought I'd post it here to get some good advice. Is this better or should I remove the post entirely?
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user3422153Mar 20 '14 at 11:31

IMHO, distinguishing between adjacent genres can be a practical, on-topic question. The primary issue is managing, and responding to, reader expectations - which affects style, marketing, classification, and other practical issues. That said, genre boundaries can be fuzzy, and so answers are likely to be guidelines rather than a clear-cut taxonomy.
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Standback♦Mar 20 '14 at 11:49

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All that being said, I think this question would be improved by editing for conciseness. As phrased, the question raises several possible answers and shoots them down in great detail; this makes the question much bulkier and more discussion-y. I would summarize to: "Many of my intuitive distinctions, such as (X, Y, Z), have obvious exceptions to them." (Possibly the text as-is would make a good answer.)
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Standback♦Mar 20 '14 at 11:54

2 Answers
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It may sound odd, but I think the main criterion is how the story treats death.

If death is one possible threat among many (being captured, being tortured, the macguffin falling to the enemy, blackmail, heartbreak, public exposure, humiliation, political scandal, strategic losses, military losses, code being broken, plague being released), I'd call it a thriller.

If death or near-fatal injury is the main threat (nurse threatening to chop off feet, having to cut off your hand to get out of a bear trap, running through the woods to get away from chainsaw-wielding maniac, serial killer, rabid dog trying to break into car, psychotic supernatural clown drowning you in sewer), I'd call it horror.

Yeah -that plus the other elements of isolation, having friends turn on you, turning into one of them (ugh!) etc. You hit the nail right on the head - getting marked for death for no other reason than the fact that you were in the wrong place at the wrong time is scary. I'm surprised at how you managed to boil the essence of the matter down into one sentence. It's all about their emotional response to death or injury.
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user3422153Mar 21 '14 at 14:53

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But you can have a horror movie in which the victim isn't isolated (Cloverfield, maybe -- IT might qualify), your friends don't turn on you (Hell Night, IT), and you don't turn into "one of them" (IT, Misery). Those are frequently seen in horror movies, but I wouldn't call any of them required criteria.
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Lauren IpsumMar 21 '14 at 18:17

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That's true. Horror comes in many different forms, and that's exactly why your answer made so much sense.
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user3422153Mar 22 '14 at 4:01

I would say that it is how the viewer or reader treats the Death. I mean, if there is a maniac that wants to kill you shoting your head with a gun. There would be no horror on it. Because, dying by being shot is something that we are used to see in movies and read in books, even in the social media nowdays.

But if you are in a dark place, or have your senses blocked by someway or something, and you know that something never aknowledge by the masses is running after you to do God knows what and then kill you in an unknow manner and alone, then I would call it horror.

It is not the simple fear of death or anything bad that could happen to you, it is the fear of something that is not seen or heard or touched or aknowledged by the masses, something that was hidden and for an unknow motive decided to appear to you and only you or to a closed group individuals. And you feel in you very soul that it is going to do something really bad to you and everyone else. Not only kill you.

I take it by the fact that there isn't horror movies depicted in large scale areas and with many people witnessing it.

But that is just my guess, and you can clearly see that I am an amateur on this.